1. Field of the Invention
The invention relates to a control apparatus for an automatic transmission.
2. Description of Related Art
There is known a control apparatus for an automatic transmission that automatically changes the speed of an output from an engine and transmits the output to driving wheels (e.g., see Japanese Patent Application Publication No. 2007-146901 (JP-2007-146901 A) and WO 2009/084294).
An automatic transmission of Japanese Patent Application Publication No. 2007-146901 (JP-2007-146901 A) is equipped with a first shifting portion (a front planetary portion) that is mainly constituted of a first planetary gear train, a second shifting portion (a rear planetary portion) that is mainly constituted of a second planetary gear train and a third planetary gear train, and a plurality of frictional engagement elements (clutches and brakes). This automatic transmission establishes each shift speed by selecting and engaging two of the plurality of the frictional engagement elements. The automatic transmission changes the speed of rotation input to an input shaft at a speed ratio corresponding to each shift speed, and outputs the rotation from an output shaft.
Besides, the respective frictional engagement elements are engaged or released through an engagement pressure that is regulated by an oil pressure control device. This oil pressure control device is provided with linear solenoid valves for regulating engagement pressures of the respective frictional engagement elements, and a failsafe gear stage switching valve for establishing a low-speed failsafe gear stage or a high-speed failsafe gear stage when there is a malfunction in at least one of the linear solenoid valves. Thus, even in the case where there is a malfunction in at least one of the linear solenoid valves, the low-speed failsafe gear stage or the high-speed failsafe gear stage can be established.
Incidentally, in WO 2009/084294, there is described a control apparatus for an automatic transmission. In this control apparatus, in the case where, with two frictional engagement elements engaged, another frictional engagement element is engaged on the basis of a line pressure, an engagement pressure is set such that one of these three frictional engagement elements slips, whereby a cutoff valve is omitted. Incidentally, a cutoff valve is designed to prevent the three frictional engagement elements from being simultaneously engaged. When an engagement pressure of that one of the frictional engagement elements which is engaged during a normal state is input to the cutoff valve, the cutoff valve serves to shut off oil pressures applied to the other frictional engagement elements.